1. Field of the Invention
The present invention relates generally to radar active array antennas and more particularly to T/R switches and low-noise amplifiers for active arrays.
2. Description of the Related Art
The face of an active array antenna for a pulsed radar system is typically partitioned into an array of transmit/receive (T/R) modules. Each of the T/R modules generally includes a power amplifier, a phase shifter, a microwave switch and a low-noise amplifier (LNA). During the transmit mode of the radar, a low-power, microwave or millimeter-wave signal pulse is fed through a distribution system to deliver a portion of the signal to each T/R module, where it is amplified by the module's power amplifier and radiated from that module's position in the antenna face. The phase of each of the radiated signal portions is selectively set by its respective module's phase shifter so that the antenna's combined radiated energy has a wavefront (a radiation surface of constant phase) with a selected orientation. Because an antenna beam is always orthogonal with its wavefront, the beam will then be steered along a selected axis.
During the receive mode of the radar, a portion of the radar echo signal is received by each T/R module and is amplified in its LNA. In addition, the phase of each receive portion is set by its respective phase shifter to selectively steer the receive antenna beam in the same manner as the transmit antenna beam was steered. The amplified echo signals of the T/R modules are then combined and routed to signal processing circuits for detection of target range and direction.
The radar's sensitivity is a function of the noise figure during the receive mode. Therefore, it is important that each module's LNA is positioned as close to the antenna face as possible to reduce the signal loss that occurs ahead of it. Accordingly, there are no frequency down-conversion circuits preceding the LNAs and they must operate at the radar pulse frequency. However, the LNAs could be damaged by leakage radiation from the transmitted pulse so it is imperative to position a protective element ahead of each LNA, typically in the form of a transmit/receive microwave switch (T/R switch). In the transmit mode, the T/R switch is set to block microwave energy away from the LNA and in the receive mode it is set to conduct microwave energy.
Noise figure at the radar's radiated frequency is the dominant parameter in the selection of transistors for the LNA. In contrast, a high voltage breakdown rating and a low on-resistance are the primary parameters considered in the selection of transistors for the T/R switch. In the absence of other considerations, a high electron mobility transistor (HEMT) and a heterostructure bipolar transistor (HBT) would generally be the transistors of choice for the LNA and the T/R switch, respectively. HEMTs have superior high frequency noise figure, e.g., 6 db at 58 GHz, and HBTs can be configured with high breakdown rating and low on-resistance.
However, the spatial limitations in an active array antenna are best met by realizing the T/R switch and the LNA as a monolithic microwave integrated circuit (MMIC). In a MMIC, all components, e.g., transistors, transmission lines, inductors, resistors and capacitors, are fabricated on the same semiconductor substrate. No bonding between components is required except to connect to the outside world. A variety of integrated circuit fabrication techniques are used, e.g., diffusion, ion implantation, oxidation and film deposition, epitaxial growth, lithography and etching and photoresist. Impedance matching networks and transmission lines are preferably realized in microstrip structures because they provide a free and accessible surface for interconnection. Resistive loads are typically deposited as thin films of an appropriate resistive material. The single-crystal semiconductor layers required to build transistors are generally grown with epitaxial processes, e.g., vapor-phase epitaxy, molecular-beam epitaxy and liquid-phase epitaxy.
A MMIC conventionally includes transistors of the same device type because each type of transistor requires a different sequence of semiconductor layers with different doping levels and thicknesses. For example, Sze, S. M. shows typical HEMT and HBT heterostructures (a heterostructure is a semiconductor structure having adjacent layers of different chemical composition, but of similar crystalline structure) that have different layer arrangements (see pages 285 and 373 of Sze, S. M., High-Speed Semiconductor Devices, Wiley-Interspace Publishing, New York, 1990). These dissimilar transistors cannot be fabricated in the same heterostructure.
Because of the difficulty of fabricating dissimilar semiconductor device types on a single MMIC, the spatial limitations of active array antennas have conventionally demanded a compromise in T/R switch/LNA performance, i.e. the selection of a single device type. This compromise is often met by fabricating the MMIC with HEMTs for realizing both the T/R switch and the LNA. In spite of the low breakdown rating of HEMTs, e.g., 5-10 volts, this compromise has usually been chosen because of the overriding importance of noise figure in the radar's sensitivity.
Future active array antenna designs envision increased transmit power levels which will require T/R modules to operate in the presence of higher leakage levels in the transmit mode, e.g., 20-40 watts. These power leakage levels will require the transistors of the T/R switch to withstand breakdown voltages in excess of 30 volts, which is far beyond the typical breakdown rating of HEMTs. In addition, improvements in HEMT breakdown voltage are typically achieved at the cost of increased noise figure, an undesirable compromise. It would be difficult to meet these future active array demands with a single semiconductor device type.